The invention relates to a laser illumination arrangement for illuminating a photosensitive layer in the course of the manufacture of a cathode ray tube, said laser illumination arrangement comprising a laser and an optical imaging arrangement for imaging laser light onto the photosensitive layer which is applied to a display window of or for a cathode ray tube.
Such illumination arrangements can be used to manufacture a cathode ray tube, inter alia, for creating phosphor patterns and black matrix patterns on a display window of a cathode ray tube.
An illumination arrangement of the type mentioned in the opening paragraph is disclosed in United States patent specification U.S. Pat. No. 4,117,177. In this patent specification, a description is given of an arrangement in which a photosensitive layer is illuminated by means of a laser beam. The photosensitive layer is applied to a surface of a display window of or for a cathode ray tube. Illumination takes place by imaging laser light onto and scanning it across the photosensitive layer. A shadow mask comprising a large number of openings is situated between the illumination arrangement and the photosensitive layer. The laser light is directed through the openings in the shadow mask at such an angle that a pattern is formed behind the shadow mask. This process is repeated a number of times, at a number of different angles, thereby creating a number of patterns. In this manner, phosphor patterns and/or black matrix patterns can be provided.
Although the use of a laser for illuminating the photosensitive layer offers a number of advantages, it has been found in practice that by using the above-described method, the illumination pattern is subject to variations, leading to an increase of the number of rejects (=the number/percentage of illuminated photosensitive layers which do not meet the quality requirements imposed and hence must be removed from the production process).
It is an object of the invention to provide an improved illumination arrangement of the type mentioned in the opening paragraph.
To achieve this, a laser illumination arrangement in accordance with the invention is characterized in that the arrangement comprises a beam homogenizer between the laser and the optical imaging arrangement or as a part of the optical imaging arrangement.
A beam homogenizer reduces the coherence of a laser beam incident on the beam homogenizer.
A laser beam is a concentrated coherent beam of light. In practice it has been found that undesirable interference patterns and reflection patterns occur. The beam of coherent laser light interferes with itself, thereby causing interference patterns. Also scattering at, for example, dust particles in the air or on an optical element, such as a lens or a mirror, occurs. These interference patterns and scattering patterns cannot be predicted. Outside and superposed on the actual beam, the patterns form a kind of fleck pattern. As a result, light of a deviating intensity, i.e. of an intensity other than the desired intensity, is incident on the photosensitive layer at locations where this is undesirable. This leads to rejects. In the laser illumination arrangement in accordance with the invention, the laser beam is controlled by a beam homogenizer. In said beam homogenizer, the coherent laser beam which is incident on the beam homogenizer is converted to a much less coherent beam. In fact, a plurality of sub laser beams are formed in the beam integrator, which jointly form a broad beam. Since the coherence is reduced substantially, or preferably has disappeared, interference patterns of the laser beam at itself occur to a much smaller degree. Reflection at dust particles may still occur, but since the beam has generally become broader, the intensity of reflected light will be smaller, and more importantly, since the coherence of the laser beam is reduced, the reflected light will generally be spread over a much larger space angle (and will thus be much less concentrated). The maximum intensity deviation which can occur at a spot of the photosensitive layer as a result of reflection at dust or other particles has been reduced by orders of magnitude.
Unlike the known state of the art, the laser beam is preferably not scanned across the photosensitive layer, but instead, the laser illumination arrangement comprises a wide-angle objective for imaging the laser beam onto the photosensitive layer. This has the advantage that moving parts are not necessary. Moving parts cause vibrations which may disturb the image or the setting of the laser illumination arrangement. In addition, moving parts are susceptible to failure and require relatively much maintenance work.
The use of a wide-angle objective enables a virtual light source to be created. Preferably the arrangement is such that the size of the virtual light source ranges between 1 and 2.5 mm. If the light source is smaller than 1 mm, the intensity of the light exhibits pronounced ripples behind an opening in the shadow mask, which is undesirable. If the size exceeds 2.5 mm, the intensity becomes lower. Preferably, the size exceeds 1.5 mm.
Preferably, the beam homogenizer is formed such that the shape of the beam issuing from the beam homogenizer is substantially identical to that of the photosensitive layer of or for a cathode ray tube. For this reason, for illuminating a 4xc3x973 display screen, use is made of a laser beam (after having been guided through the beam homogenizer) with a length/width ratio of approximately 4xc3x973; for a 16xc3x979 display screen, use is made of a laser beam having a length/width ratio of 16xc3x979 etc. By virtue thereof, the illumination efficiency (the amount of light used) can be increased.
These and other aspects of the invention will be apparent from and elucidated with reference to the embodiment(s) described hereinafter.